In recent years, a liquid crystal display has become bigger and has been used for home wall-mounted televisions as well as personal notebook computers, accordingly, it is required to ensure the high definition, the high quality, and the wide viewing angle in respects to the liquid crystal display. In particular, since a thin film transistor liquid crystal display (TFT-LCD) that is driven by using a thin film transistor independently drives each of pixels, a response rate of the liquid crystal is very high, thus, a high-quality dynamic image can be realized. Accordingly, currently, the application range thereof is expanded.
In the TFT-LCD, in order to use the liquid crystal as an optical switch, it is required that the liquid crystal is initially aligned on the layer on which the thin film transistor is formed at the innermost part of the display cell in a predetermined direction. In order to achieve this, a liquid crystal alignment film is used.
With respect to a liquid crystal display, currently, a process of aligning a liquid crystal includes applying a heat resistant polymer film such as polyimide on a transparent glass to form a polymer alignment film and rubbing the alignment film while a rotation roller that is wound by a rubbing cloth made of nylon, rayon or the like is rotated at a high rate to achieve the alignment. This process is called a rubbing process.
However, since the rubbing process may cause mechanical scratches or the high static electricity on the surface of the liquid crystal alignment material during the rubbing process, a thin film transistor is destroyed and inferiority occurs due to fine fibers generated from the rubbing cloth, thus preventing the production yield from being improved. In order to avoid the problem of the rubbing process to improve the productivity, a novel liquid crystal alignment process is designed, which is a liquid crystal alignment by using UV, that is, light (hereinafter, referred to as “optical alignment”).
The optical alignment is a mechanism in which a photoreaction of a photosensitive group that is connected to the polymer occurs due to linearly polarized ultraviolet rays, and in this procedure, a main chain of the polymer is unidirectionally aligned, thereby forming a photopolymerizable liquid crystal alignment film in which the liquid crystals are aligned.
A representative example of the optical alignment is an optical alignment using photopolymerization that is announced by M. Schadt, et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang, et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000).
The polycinnamate-based polymer such as PVCN (poly(vinyl cinnamate)) and PVMC (poly(vinyl methoxycinnamate)) has been mainly used as the optical alignment polymer that is disclosed in the known patent documents and papers. In the case of when the polycinnamate-based polymer is subjected to the optical alignment, the double bond [2+2] of cinnamate is subjected to the [2+2] cycloaddition reaction by using radiated UV to form cyclobutane, thus, an anisotropic property is formed to unidirectionally align liquid crystal molecules, which causes the alignment of the liquid crystals.
In the related art, in respects to the optical alignment polymer, Japanese Unexamined Patent Application Publication No. 11-181127 discloses a method of producing a polymer type of alignment film that has a main chain such as acrylate and methacrylate and a side chain having a photosensitive group such as a cinnamate group, and an alignment film that is produced by using the method. However, the patent is disadvantageous in that since the mobility of the polymer is poor, even though the polymer is exposed to light for a long time, it is difficult to obtain the desired alignment property. The reason for this is that since the photosensitive group which is present in the polymer is restricted by the main chain of the polymer, the group is difficult to rapidly react with the radiated polarized light. Accordingly, since a long time is required to obtain a network polymer, a process efficiency is reduced, and if an alignment treatment process is finished after an insufficient time, since the alignment of the liquid crystals is insufficient in the manufactured liquid crystal display, there are problems in that a dichroic ratio is low and contrast is reduced.